The invention relates to ultraviolet light absorbers useful as stabilizers for plastics and coating compositions. More particularly, the invention concerns coupled compounds which contain both benzotriazole and benzophenone parts, which are u.v. light stabilizers.
A problem in the art of manufacturing and processing thermoplastic polymers and coating compositions is their instability upon extended exposure to ultraviolet light sources. It is well known that light, oxygen and heat cause degradation of polymers resulting in deterioration of mechanical and physical properties of the polymer. Coatings and plastics tend to demonstrate unwanted color changes and reduced mechanical strength upon exposure to uv radiation. For example, in the case of polyvinyl chloride, it is theorized that ultraviolet exposure results in the formation of free radicals on the polymer chain which react with atmospheric oxygen. The resulting peroxide groups are in such a position that they activate a chlorine atom giving rise to a weak spot on the polymer. Irradiation in the presence of oxygen causes the molecular weight to decrease and is indicative of chain scission. Irradiation in the absence of oxygen causes an increase in molecular weight due to crosslinking. It is well known that polymer exposure to ultraviolet radiation will cause discoloration and brittleness, thus destroying its physical properties. To prevent or at least retard the damage caused by these factors, stabilizers are added to the plastic.
Various attempts have been made to improve resistance to deterioration by the introduction of uv stabilizers which are radiation absorbing. Light stabilizers are compounds which when added to the polymer are capable of interfering with the reactions of degradation caused by light energy. There are several classes of compounds which are used commercially for this purpose, the more important being 2-hydroxyphenyl benzotriazoles and benzophenones. It is generally accepted that 2-hydroxyphenyl benzotriazoles and benzophenones function as u.v. light stabilizers through the hydrogen bonding of the ortho hydroxy phenyl groups absorbing photons and dissipating it as harmless heat energy. There are many patents covering the manufacture and use of these u.v. absorbers in polymers and coatings. Both classes of compounds are themselves stable to both heat and light during long term exposure.
Although many stabilizers are known in the art, none are completely satisfactory in performance. The absorption of light by polymers is dependent on its chemical structure. Saturated plastics do not absorb above 250 nm. However, the presence of double bonds or chromophores causes the absorption to shift to higher wavelengths. Carbonyl structures absorb above 290 nm. The presence of impurities such as catalyst residues, oxidation by-products, plasticizers and other stabilizers may cause the polymer to absorb at higher wavelengths. U.V. light is also capable of breaking chemical bonds in polymers which changes the absorption characteristics of the polymer. Polymers are degraded by radiation with wavelengths of less than 325 nanometers. Threshold u.v. wavelengths for various chemical bonds below which bonds will break are C--N at 392.7 nm, C--Cl at 352.0 nm, C--C at 346.1 nm, S--H at 344.5 nm, N--H at 336.4 nm, C--O at 334.4 nm and C--H at 289.7 nm. The wavelength regions for the u.v. light types are UV-A at 320 to 400 nm which causes polymer degradation, causes suntan but no sunburn, and is transmitted by window glass. UV-B at 280 to 320 nm, which has the shortest wavelengths at the surface of the earth, is responsible for the most polymer damage, causes sunburn and is absorbed by window glass. UV-C, below 280 nm is found only in outer space or as produced by artificial light sources. It can be filtered out by ozone in the atmosphere and can cause abnormal reactions. Degradation also takes place at higher wavelengths but to a lesser degree. Commercial ultraviolet absorbers absorb radiation in a range between 290 and 400 nanometers. A definite yellowing of the polymer results when the ultraviolet absorber absorbs above 400 nanometers. The activation spectra for various polymers and the wavelengths of maximum damage are: polyvinyl acetate film&lt;280 nm; polycarbonate film at 285 nm, 305 nm, 330 nm and 360 nm; acrylics at 290 nm and 325 nm; SAN film at 290 nm and 325 nm; CAB film at 295 nm and 298 nm; polyethylene at 300 nm; polypropylene at 310 nm and 370 nm, PVC at 320 nm; polyester at 325 nm and PVC/vinyl acetate at 327 nm and 364 nm.
There are other considerations when choosing a suitable u.v light stabilizer. Compatibility is one of them. Many stabilizers prove to be incompatible with the polymer or coating composition being stabilized. This is usually a function of its solubility in the polymer and can be indirectly measured by determining the solubility of the polymer in organic solvents. The higher the solubility in organic solvents, the greater the probability of compatibility in the polymer. Incomplete or poor solubility causes a lower absorbance than the theoretical value. It also results in exudation of the u.v. light absorber to the polymer surface. Another important consideration is the volatility of the stabilizer in the polymer and during processing. The stabilizer must have sufficiently high molecular weight so that it remains in the polymer matrix for the life of the plastic. Many polymers are processed at very high temperatures which causes the stabilizer to vaporize. Not only is some of the stabilizer lost to the atmosphere and not available to the polymer, but the working conditions require workers to wear gas masks and protective gear during processing. This creates a serious health hazard. Manufacturers have sought to solve the volatility problem by increasing the molecular weight of the u.v. absorber. To improve the solubility and reduce the volatility, appropriate side chains have been added to the u.v. light absorber molecule. This was usually done by adding various chemical groups to the phenolic ring or the benzene ring of the benzotriazole and benzophenone. In most cases, the additional molecular weight does not contribute to the light absorption properties of the u.v. light stabilizer. It does not enhance the ability to change the absorbed photons to harmless dissipated heat. Since such groups usually do not contain chromophores, the u.v. absorption properties of the modified u.v. light absorber not only are not enhanced, they actually reduce the absorption. The additional problems of the rate of diffusion through the composition and the rate of loss of the stabilizer from the composition must be empirically determined and are not generally predictable.
In particular, it is known that some 2-hydroxy-4-alkoxybenzophenones are useful uv absorbers and light stabilizers. U.S. Pat. Nos. 3,399,237 and 3,310,525 show compounds having a plurality of benzophenone functional groups. U.S. Pat. No. 4,186,151 mentions symmetrical and asymmetrical 5,5'-methylenebis(2-hydroxy-4-alkoxybenzophenone) compounds, but it cites no examples of asymmetrical compounds. Hydroxyphenyl benzotriazole monomers and dimers are also known to be uv absorbing. Monomers are disclosed in U.S. Pat. Nos. 5,097,041, 4,943,637 and 5,104,992. U.S. Pat. Nos. 4,684,679; 4,937,348 and 4,812,498 disclose symmetrical hydroxyphenyl benzotriazole dimers. All of these patents are incorporated herein by reference.
There are only a relatively few benzophenones and benzotriazoles which are effective stabilizers for plastics and coatings. All have very different properties with regard to solubility, melting point, molecular weight, uv absorption, and polymer compatibility and therefore, it is often impossible to select a given benzophenone or benzotriazole having the best combination of properties for the polymer or coating composition of choice. While it may be desired for a given stabilizer to have a certain uv absorption characteristic or melting point, it may not be compatible with the plastic preference. The hydroxyphenyl benzotriazoles possess the more desirable overall absorption characteristics since they absorb very strongly throughout most of the u.v. region and very little as the wavelength approaches 400 nm.
It has now been found that the volatility of benzotriazole and benzophenone stabilizers can be sharply reduced by coupling different u.v. light absorbing chemical moieties by utilizing methylene bridge processes. This route is highly selective in producing u.v. light stabilizers in good yield and purity. It has been unexpectedly found, that by forming a coupled compound which has 2-hydroxy-4-alkoxybenzophenone and hydroxybenzotriazolylphenol moieties, an improved class of uv stabilizers is produced. Surprisingly, the coupled benzotriazole-benzophenone u.v. light stabilizers resulting for this invention have lower melting points, better solubility in organic solvents, and better compatibility in most polymers than could have been predicted. In addition, the u.v. light absorption spectrum of the products covers a wider wavelength band than either the benzotriazole or benzophenone alone. This opens the possibility to tailor make u.v. light stabilizers having absorption qualities fitted to the needs of the particular polymer. The melting point, molecular weight, and uv absorption properties can be adjusted to the requirements of the polymer of choice, since each moiety contributes its beneficial characteristics. It has also been found that in general, such coupled compounds are more soluble in organic solvents than the symmetrical dimers made from the same monomer constituents. As a result, they are more easily and more uniformly blended in the plastic or coating composition to be stabilized.